There are several Id like to be:
Celebi- The fact it can go back in time is pure awesomeness. All of my life questions could be aswered if I could go back to see the even again. Also psychic type is awesome. And it is SOOOOOO cute. Also the fact that if you hurt yourself badly, you can use the move recover to not die.
Squirtle- Such a cute turtle pokemon. The fact that it can learn Withdraw to defend itself is also great. And due to the fact that it is one of the worlds fastest swimmers, Id love to swim the wide sea looking for surprises at every corner. The only downside is that they are heavily captured for starters, unless I could be a wild squirtle
Eevee- yes even though I love eseon, I think Id like to be an eevee more, who knows into what I could evolve, I mean its like 6 choices. Maybe I could train at that place for a lefeon (btw does anybody think that skitty is the new eevee?)
Charmander- eh.... the thought of evolving into some monstrous beast draws me in
Larvitar- same as charmander
Teddiursa- OMG teddiursa is soooooo cute. I mean its like a teddy bear. If I were to evolve into ursaring, I would cancel my evolution due to the fact that I like teddiursa better than ursaring.
Caterpie- I would love to evolve into a butterfree. The thought of being so majestic is so awesome. At night when I fly around with my flock of butterfree spraying sleep powder all over the place. Plus Id be great for contests (even better than beautifly actually). And how I can just fly all over the pokemon world, meeting new pokemon, and perhaps getting meself a shiny mate just like Ash's butterfree did